The present invention relates generally to methods and systems for dispensing fuel gases, such as compressed natural gas (CNG), to a storage vessel on a vehicle. More specifically, embodiments of the present invention relate to systems and methods for ensuring that a fuel gas storage vessel is completely filled during dispensing, regardless of ambient temperature or the initial pressure of the storage vessel.
Those of skill in the art are familiar with dispensing systems for vehicles that are fueled by gas that is flowed into a cylinder or vessel, such as natural gas vehicles (NGVs). In general, NGV storage vessels are rated to be completely filled with gas under a given pressure at a given temperature (e.g., 3600 psi at 70° F.). The Ideal Gas Law defines a direct relationship between the temperature and pressure of a gas being added into a fixed-volume storage vessel, and thus as the temperature of the gas increases, greater pressures are required to completely fill the vessel. As the gas inside the vessel cools, the pressure inside the vessel decreases.
Most filling systems are configured to terminate when the dispenser measures a target pressure of the gas in the NGV storage vessel at which it is expected that the vessel should be completely filled. In the past, the target pressure has been based on the pressure at which the storage vessel is rated to be completely filled at the measured ambient temperature. In addition, as a safety measure, the filling system is typically configured to halt dispensing if the pressure in the vessel reaches a predetermined cutoff pressure. This cutoff pressure is typically greater than the target pressure but lower than the maximum pressure the vessel is designed to safely accommodate.
As is known, prior art dispensing systems for CNG, including those that compensate for differences in ambient temperature, often fail to completely fill, or “charge,” the vessels, especially at higher ambient temperatures. In particular, during a “fast-fill” dispensing process (i.e., one that is typically designed to take fewer than five minutes), it is often the case that a pressure greater than the cutoff pressure is required to completely charge the NGV's storage vessel. As a result, the dispenser will terminate the filling process when the gas reaches the cutoff pressure, before the vessel is completely charged.
Those of skill in the art are familiar with a variety of prior art systems which attempt to solve the problem described above. Examples of such attempts include U.S. Pat. Nos. 5,259,424; 5,641,005; 5,653,269; 5,868,176; and 5,881,779, the disclosure of each of which is incorporated by reference herein in its entirety for all purposes. Unfortunately, however, prior art attempts have failed to solve this problem, and they cannot accurately determine when the storage vessel will be completely filled to its rated capacity, yet not overfilled.